This application incorporates by reference Taiwanese application Serial No. 89117550, filed on Aug. 29, 2001.
1. Field of the Invention
The invention relates in general to an apparatus for temperature sensing and heating, and more particularly to an apparatus for temperature sensing and heating for use in a print head.
2. Description of the Related Art
Over the years, electronic related industries progress as the technology advances. For various electronic products, such as computer systems, computer peripherals, appliances and office machines, their functions and appearances are improved greatly as well. For example, in the 1980s, impact-type dot matrix printers and monochrome laser printers were pre-dominant. Later in the 1990s, monochrome inkjet printers and color inkjet printers became popular for common uses while color laser printers were available for professional uses. For common end users who do not print documents frequently, they would probably select color inkjet printers after considering the printing quality and price. People with sufficient budgets would probably purchase a monochrome laser printer. Since the price and quality are critical to the users"" choices, printer vendors aggressively develop their products so that the products have lower cost and better quality so as to increase popularity and profits of their products. Therefore, developers are focusing on how to improve the performance of products under limited cost.
Most inkjet printers now use bubble inkjet print head or piezo-electrical inkjet print head to spray ink droplets onto a sheet of medium, such as paper, for printing. The bubble inkjet print head includes a heating device, ink, and nozzles. The heating device is to heat the ink to create bubbles until the bubbles expand enough to burst so that ink droplets are fired onto the sheet of paper through the nozzles, forming dots on the sheet of paper. Varying the concentration and locations of the droplets can form wide range of different texts and graphics on the paper.
The quality of printing is closely related to the resolution provided by the printers. Currently, entry-level color printers provide a maximum resolution of 720 by 720 dot per inch (dpi) or 1440 by 720 dpi. Higher resolution requires finer size of the droplets. The size of the droplets is related to the cohesion of the droplets. For instance, for droplets having identical amount of ink, those droplets with greater cohesion may have a smaller range of spread when they fall onto the paper, resulting in clearer and sharper printing quality. On the other hand, those droplets with smaller cohesion may have a greater range of spread when they fall onto the paper, resulting in a poorer printing quality. Thus, cohesion of the droplets affects the printing quality. In common bubble inkjet printing technique, if it is required to eject ink droplets by a specific nozzle, the heating device associated with the nozzle is first enabled to heat the ink so as to generate bubbles in the chamber associated with the nozzle. The viscosity of the ink decreases as the temperature of the ink rises. If the heating process is not well controlled and the ink is overheated, the viscosity of the ink becomes lower than a normal level and the cohesion of the droplets is reduced, resulting in a degraded printing quality. In addition, if the chamber contains insufficient ink or the ink droplet is not fired properly, the temperature of the ink in the chamber will exceed the normal level, resulting in the viscosity of the ink being lower than the normal. In addition, if a nozzle is frequently fired, the ink in the chamber associated with the nozzle will have higher temperature and lower viscosity than the ink in the chamber associated with other nozzles. All these conditions cause the viscosity of the ink to be unstable, and thus affecting the printing quality. Therefore, accurately monitoring and controlling the temperature of the ink in the chamber is the key to the improvement in the ink jet printing quality.
FIG. 1A is a block diagram illustrating the conventional control of an inkjet printer. The inkjet printer 10 includes a driving module 11 and a print head module 15. The driving module 11 includes a controller 12 and a driver circuit 13. The print head module 15 includes an array of inkjet ejector 16 and a temperature sensing device 17. For the printing of data onto a sheet of paper, the controller 12, in response to the data, drives the driver circuit 13 so that the driver circuit 13 sends selection signals 14 to the array of inkjet ejectors 16. In the array of inkjet ejectors 16, selected heating devices such as a heating device 19 shown in FIG. 1B heat up according to selection signals 14 so that ink droplets are ejected onto the paper through the nozzles of the array of inkjet ejectors 16. FIG. 1B is a sectional view illustrating the array of inkjet ejectors 16 shown in FIG. 1A along with the heating device 19 and a nozzle 18. The heating device 19 is mounted in close proximity to the nozzle 18, and is used for heating the ink in the chamber 21 in order to create a bubble 20. The ink in the chamber 21 is heating up until the pressure in the chamber 21 forces the bubble 20 to burst and a droplet of ink is ejected from the nozzle 18. The ejected ink droplet then forms a spot on the sheet of paper.
Further, in order to monitor the temperature of the nozzles, a temperature sensing device 17, such as a thermal resistor, is arranged near a portion of nozzles of the array of inkjet ejectors 16. The measured temperature data from the temperature sensing device 17 is fed back to the controller 12 for the control of the temperature.
In the following, it is to describe how to select heating devices according to selection signals 14 so that ink droplets are ejected from the nozzles. FIG. 2 is a circuit diagram illustrating the array of inkjet ejectors 16 in FIG. 1A. The array of inkjet ejectors 16 includes an Mxc3x97N two-dimensional array of circuit elements. Each of the circuit elements is formed by a resistor R coupled with a transistor Q, and is associated with one of the nozzles. Besides, the selection signals 14 are selectively applied to the circuit units to create bubbles and cause ink droplets to be ejected for the formation of marks on the sheet of paper. When one of the selection signals 14 is selectively applied to the circuit element to cause the transistor Q conduct, the resistor R generates heat for the ink of the chamber 21 to cause a ink droplet to be ejected from the nozzle 18. In other words, the resistor R is used as the heating device for heating the ink of the chamber. In addition, for the reduction of the number of signals, the selection signals can be composed of row signals and column signals. In FIG. 2, Xa denotes one row signal of the selection signals 14 while Yb denotes one column signals of the selection signals 14, where a=1, 2, . . . , M and b=1, 2, . . . , N. For the sake of brevity, this notation will be used in the following of the specification. For instance, when the row signal X1 and column signal Y1 are active and fed to the array of ink ejectors 16, the transistor Q11 conducts and thus the resistor R11 produces heat so that a droplet of ink is ejected from the associated nozzle. Likewise, when the row signal XM and column signal YN are active and fed to the array of ink ejectors 16, the transistor QMN conducts and thus the resistor RMN produces heat so that a droplet of ink is ejected from the associated nozzle. In this way, according to the row and column signals of selection signals 14, the nozzles indicated by selection signals 14 can be accurately enabled for printing.
FIG. 3 are comparative graphs of measured temperature of the nozzles in the same structure as in FIG. 1B versus the time as the nozzles are in a normal case and in an abnormal case. In the normal case, the temperature of the nozzles increases as the ink is being heated and then it reduces after the ejection of ink occurs. The temperature variation in the normal case can be represented by the curve denoted as xe2x80x9cnormal nozzlexe2x80x9d. In the abnormal case, such as the blockage in some nozzles, the ink droplets cannot be produced and the heat cannot dissipate, resulting in a small reduction of the temperature of the nozzles. The temperature variation in this abnormal case can be represented by the curve denoted as xe2x80x9cabnormal nozzlexe2x80x9d.
In the conventional print head module 15 shown in FIG. 1, the temperature of the nozzles is obtained from the temperature sensing device 17 which is formed by a thermal resistor arranged near some of the nozzles. In addition, the temperature of the nozzles is determined by the variation of the resistance of the thermal resistor.
However, the temperature obtained in this way is an average temperature of some or all of the nozzles whereas the change of the temperature of one of the nozzles is unobtainable. Therefore, if the temperature of one or a small number of nozzles increases abnormally, the temperature sensing device 17 of the conventional print head module 15 cannot determine which nozzle has an abnormal increase in temperature and the temperature compensation for this abnormal increase in temperature may be inadequate.
It is therefore an object of the invention to provide a print head apparatus capable of sensing the temperature of nozzles selectively.
It is another object of the invention to provide a print head apparatus capable of sensing the temperature of nozzles selectively or heating the nozzles selectively, which can be applied to the design of a system without the substantial changes in the design.
According to the objects of the invention, it provides a print head apparatus capable of temperature sensing. The print head apparatus includes an ink ejector coupled to an enabling signal and a selection signal for selecting the ink ejector. The ink ejector includes a nozzle, a heating module for selectively heating ink in the ink ejector so that ink droplets are ejected from the nozzle, and a temperature sensing module for selectively producing a measured temperature signal indicative of a temperature of the ink in close proximity to the nozzle. The heating module includes a heating device and an enabling gate. The heating device is coupled to the enabling gate and is disposed in close proximity to the nozzle for heating up the ink in the ink ejector in order to eject ink droplets from the nozzle. The enabling gate is coupled to the enabling signal and is used to cause the heating device to heat up. The temperature sensing module includes a temperature sensor and a detection gate. The temperature sensor is disposed in close proximity to the nozzle and coupled to the detection gate, and is used for measuring the temperature of the ink in close proximity to the nozzle and producing the measured temperature signal indicative of the temperature of the ink in close proximity to the nozzle. The detection gate is coupled to the selection signal, and is used for selectively outputting the measured temperature signal. When the selection signal is active and indicates that the ink ejector is selected, the temperature sensing module outputs the measured temperature signal indicative of the temperature of the ink in close proximity to the nozzle.